@blue-cat I imagine this best being accomplished through the reputation system. If one were to create a category called "Introductions" that did not require any reputation to post, but all other categories required some reputation, then new users would only have the option to post in the "Introductions" category. However, from what I can tell there is not currently the option to require minimum reputation before posting to categories. I hope someone has a more usable answer.

Not avaiable out of the box, but you can use something like https://github.com/julianlam/nodebb-plugin-archiver or modify it to delete the topics instead of locking.